Behold, Here's Poison by Georgette Heyer was first published in the 1930s and has stood the test of time extremely well. Gregory Matthews - by all accounts a dislikeable character - is murdered. At first his GP is prepared to sign a death certificate as he had been treating him for a hear condition but the corpse's family - or rather one of his sisters - wants a post mortem. It turns out he was poisoned with nicotine - how it was administered is the biggest part of the mystery with just about all his relatives and acquaintances having means, motive and opportunity.
This is a traditional country house murder mystery of the type which was so popular in the Golden Age of detective fiction. The solution is ingenious and not at all obvious with many red herrings along the way. If you leave out excessive gore - which many modern writers seem unable to do - there has to be excellent plotting and characterisation and these there are in spades in this book. Still worth reading more than 70 years after first publication.